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Page 401
same time.
(Reading or writing requires the controller to move bits on a microsec-
ond time scale, so one transfer uses up most of its computing power.) The situa-
tion is different for hard disks with integrated controllers, and in a system with
more than one of these hard drives they can operate simultaneously, at least to the
extent of transferring between the disk and the controller’s buffer memory. Only
one transfer between the controller and the main memory is possible at once, how-
ever. The ability to perform two or more operations at the same time can reduce the
average access time considerably.
Figure 5-18 compares parameters of the standard storage medium for the origi-
nal IBM PC with parameters of a disk made three decades later to show how much
disks changed in that time.
It is interesting to note that not all parameters have im-
proved as much. Average seek time is almost 9 times better than it was, transfer
rate is 16,000 times better, while capacity is up by a factor of 800,000. This pattern
has to do with relatively gradual improvements in the moving parts, but much
higher bit densities on the recording surfaces.
IBM 360-KB floppy disk
WD 3000 HLFS hard disk
Number of cylinders
Tracks per cylinder
Sectors per track
63 (avg)
Sectors per disk
Bytes per sector
Disk capacity
360 KB
300 GB
Seek time (adjacent cylinders)
6 msec
0.7 msec
Seek time (average case)
77 msec
4.2 msec
Rotation time
200 msec
6 msec
Time to transfer 1 sector
22 msec
Figure 5-18.
Disk parameters for the original IBM PC 360-KB floppy disk and a
Western Digital WD 3000 HLFS (‘‘Velociraptor’’) hard disk.
One thing to be aware of in looking at the specifications of modern hard disks
is that the geometry specified, and used by the driver software, is almost always
different from the physical format.
On old disks, the number of sectors per track
was the same for all cylinders. Modern disks are divided into zones with more sec-
tors on the outer zones than the inner ones.
Fig. 5-19(a) illustrates a tiny disk with
two zones. The outer zone has 32 sectors per track; the inner one has 16 sectors per
track. A real disk, such as the WD 3000 HLFS, typically has 16 or more zones,
with the number of sectors increasing by about 4% per zone as one goes out from
the innermost to the outermost zone.
To hide the details of how many sectors each track has, most modern disks
have a virtual geometry that is presented to the operating system. The software is
instructed to act as though there are
heads, and
sectors per track.

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